Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuits. Sputtering was developed to deposit planar metal layers used for interconnects and commercial sputtering typically utilizes a plasma of a sputter working gas, usually argon, to bombard the negatively biased target with argon ions to sputter atoms of the target material, which thereafter coat the wafer with a layer of the target material. In reactive sputtering of metal nitrides, nitrogen gas is additionally admitted into the sputtering chamber.
More recently, sputtering has been adapted to deposit thin layers of metal and metal nitride onto the walls of high-aspect ratio holes such as vias and trenches. Sputtering is widely used to deposit barrier layers of a refractory metal such as tantalum and the refractory nitride on via walls to prevent the diffusion of copper from the via into the surrounding dielectric layer of silicon oxide or low-k dielectric material. It is also used to deposit a thin copper seed layer onto the via walls to act as a plating electrode and a nucleating layer for copper filled into the via hole by electrochemical plating (ECP).
In plasma sputtering as typically practiced commercially, a target of the material to be sputter deposited is sealed to a vacuum chamber containing the wafer to be coated. Argon is admitted to the chamber. When a negative DC bias of several hundred volts is applied to target while the chamber walls or shields remain grounded, the argon is excited into a plasma. The positively charged argon ions are attracted to the negatively biased target at high energy and sputter target atoms from it. An exemplary target 10, illustrated in the schematic cross-sectional view of FIG. 1, is arranged around a central axis 12 of the vacuum chamber. The target 10 often includes a backing plate 14, which may be composed of brass, to which is bonded a target layer 16, for example of copper or tantalum. However, unitary copper targets are possible. A magnetron 18 positioned in back of the target 10 includes two opposed magnetic poles 20, 22 magnetically coupled at their back through a magnetic yoke 24 to project a magnetic field B in front of the target layer 16. The magnetic field traps electrons of the plasma and hence increases the density of the plasma to thereby increase the sputtering rate. The magnetron 18 is typically offset from the central axis 12 of the target 10 but is rotated about it to increase the azimuthal deposition uniformity. Commercial magnetrons have a more complicated shape than that illustrated in FIG. 1.
The adaptation of sputtering to coating the walls of high aspect-ratio holes has rested in part on decreasing the size of the magnetron so that the target power is concentrated in a smaller area and thus produces a higher target power density, thereby increasing the density of the plasma to a level such that a sizable fraction of the sputtered atoms are ionized. If the wafer is electrically negatively biased, the sputtered ions can be drawn deep within narrow holes in the wafer. It has been found that sputtering into high aspect-ratio holes with a high ionization fraction of sputtered atoms is promoted by primarily scanning the small magnetron 18 near the periphery of the target 10. The sputtered ions nonetheless diffuse towards the center.
Although in the past, magnetrons were designed for uniform target erosion, ionized sputtering with a small magnetron may preclude uniform erosion. Further, if only the outer portion of the target is sputtered, some of the sputtered atoms redeposit near the target center and build up an increasing thickness of redeposited material, which does not adhere well to the target and is likely to flake off and produce an unacceptable number of particles which fall on the wafer and introduce defects in the final devices. To counter this problem, Hong et al. disclose in U.S. Pat. No. 7,018,515 a magnetron that pivots between a radially outer position used for most of the sputter deposition and a radially inner position in which the magnetron cleans the target.
Care must be taken to account for erosion of the target. The peripheral scanning of the small magnetron 18 erodes the target layer 16 to produce an annular groove 26 while the central portion of the target layer 16 is eroded less below its original surface 28. Excessive sputtering of the target 10 will punch through the target layer 16 to sputter the exposed underlying backing plate 14 and contaminate the chamber especially if a tantalum barrier layer is being sputter deposited. In the case of a unitary target, excessive sputtering may mechanically weaken the target to the point where it cannot stand off the vacuum. In response to these problems, erosion may be accounted for by measuring the erosion in a test as a function of lifetime target energy, for example, the number of kilowatt-hours (kW-hr) applied to the target during operational sputtering. The lifetime target energy is experimentally established for a combination of target and magnetron as the target energy at the end-point of target sputtering, for example, just before the target layer is sputtered through to expose the backing plate. For a fixed combination of magnetron and target configuration, the measured endpoint target energy dictates when the target should be replaced.
Erosion causes a further problem in plasma sputtering. As the target 10 is eroded, the sputtering surface within the target layer 16 recedes and comes closer to the magnetron 18 so that the magnetic field at the sputtering surface changes over the lifetime of the target 10. The sputtering rate depends on the magnitude of the magnetic field B adjacent the sputtering surface, which increases with the depth of erosion. Also, the plasma may become unstable under changes of magnetic field, possibly extinguishing or sparking, the latter of which can create damaging particulates. Hong et al. in US patent application publication 2005/0133365 disclose the advantage of periodically adjusting the spacing between the target and the magnetron to compensate for long-term erosion of the target over many wafer cycles and to thereby regularize the sputtering plasma.
Miller et al. describe in U.S. Pat. No. 6,852,202 a planetary gear mechanism which achieves somewhat similar results to the two-position magnetron of U.S. Pat. No. 7,018,515 by scanning a small magnetron in a multi-lobed pattern across back of the target. Miller et al. describe a more general epicyclic scan mechanism, referred to as a universal magnetron motion (UMM) mechanism, in U.S. patent application Ser. No. 11/924,573, filed Oct. 25, 2007, now published as U.S. patent application publication 2008/0060938, and incorporated herein by reference for the details of the scanning mechanism. While the scan pattern for the planetary gear mechanism is fixed by the gears and arms forming the mechanism, the UMM mechanism can produce a nearly arbitrary scan pattern by the independent rotations of two coaxial shafts.